A geographic information system (GIS) is a computer-based tool for mapping and analyzing things that exist and events that happen on earth. GIS technology integrates common statistical analysis with the unique visualization and geographic analysis benefits offered by maps." -ESRI
In other words, a GIS is a database with which you can query and analyze multiple datasets. A GIS database is a combination of many things in order for it to be successful. You need proper software and enough storage space and processing speed on your computer to handle the vast amounts of data. Then, you'll need to spatial data itself. And don't forget continual training to stay up to speed with the latest news in the GIS world.
The most basic function of a GIS is to take tabular data and produce a visual representation of that data. As illustrated in the example below, software for GIS allows computers to take many different data layers, combine them, and then act as a cookie cutter to extract only the requested information from those layers. By combining datasets, a GIS assists people in getting a clearer picture of what they want.